Origin of Carbon

Why in news?

A study on ‘white dwarfs’ has provided new insights on the origins of the carbon in the Milky Way galaxy.

What are white dwarfs?

White dwarfs are the dense remnants of a star after its death, whose nuclear energy supplies have been used up.
They consist of degenerate matter with a very high density due to gravitational effects.

What is the importance of carbon?

Carbon is essential for life.
It is the simple building block of all the complex organic molecules that organisms need.
It is known that all the carbon in the Milky Way came from dying stars that ejected the element into their surroundings.
However, there is a debate on what kind of stars made the major contribution.

How does carbon come from stars?

Most stars, except the most massive ones, turn into white dwarfs.
When the massive ones die, they go with a spectacular bang known as the supernova.
Both low mass and massive stars eject their ashes into the surroundings before they end their lives.
These ashes contain different chemical elements, including carbon.

How carbon is released?

In both the type of stars, carbon is synthesised in its deep and hot interiors through the triple-alpha reaction.
[Triple-alpha reaction = Fusion of three helium nuclei]
In low-mass stars, the newly synthesised carbon is transported to the surface from the interiors via gigantic bubbles of gas.
From the surface, the carbon is injected into the cosmos through stellar winds.
Massive stars enrich the interstellar medium with carbon before the supernova explosion, when they also experience powerful stellar winds.

What did the study find?

In 2018, the researchers analysed a few white dwarfs belonging to open star clusters of the Milky Way.
They measured the masses of the white dwarfs, derived their masses at birth, and from there calculated the “initial-final mass relation”.
[Initial-final mass relation is a key astrophysical measure that integrates information of the entire life cycles of stars.]
They found that the more massive the star at birth, the more massive the white dwarf left at its death.
So far, stars born roughly 1.5 billion of years ago in our galaxy were thought to have produced white dwarfs about 60-65% the mass of Sun.
Instead, they were found to have died leaving behind more massive compact remnants, about 70-75% solar masses.

What explains this?

The stripping of carbon-rich outer mantle of these stars occurred slowly.
This is slow enough to allow the central cores of these stars, the future white dwarfs, to grow considerably in mass.
By analysing the initial-final mass relation, the size range for the stars that contributed carbon to the Milky Way was concluded.
Stars more massive than 2 solar masses contributed to the galactic enrichment of carbon.
Stars less massive than 1.65 solar masses did not spread its carbon-rich ashes upon death.

Having fixed the minimum initial mass for the production of carbon in low-mass stars is great, since it helps putting the pieces together.

Source: The Indian Express